Hunt will be on in 2017, but finding the source may still be a challenge.

In the Newtonian view of the world, binary star systems should remain in a stable orbit in perpetuity, no matter how massive the objects or how close the orbit. But with general relativity, that changes; energy gets carried away from the system in the form of gravity waves, which gradually causes the orbit to decay, ultimately leading to a merger.

By observing binary systems of massive objects, we've determined that general relativity gets it right. These systems behave just as general relativity predicts, giving us confidence that the theory is correct. What's missing is the other half of the confirmation: gravity waves. We haven't detected any originating from these systems. In fact, we haven't detected any, period.

It's not for lack of trying. For nearly a decade, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, searched for gravity waves from astronomical events, like the merger of two black holes (a number of other detectors have also engaged in the search, but all have come up empty). Now, scientists are readying a worldwide network of LIGO-like detectors that should start coming online in 2017. A short perspective in Science outlines the project's plans.

LIGO's design is very simple. Laser beams are sent down long, perpendicular tunnels where they bounce off mirrors before heading back to the source. When they return, the light from the two beams is allowed to interfere, which creates a distinctive pattern. If a gravity wave passes through the observatory, it will distort the space occupied by this setup ever so slightly and, since the two beams are perpendicular, the effect will be different on each. As a result, the beams will interact differently, and the interference pattern will change.

To increase sensitivity, the LIGO project also operated two detectors, one in Louisiana, the other in Washington. Due to the large separation, the waves should arrive at each at a slightly different time, allowing some rough estimate of the direction to the event that produced them.

If LIGO didn't detect anything, there were two potential explanations. Even the most energetic events, like the merger of two black holes, produce very weak gravity waves. As a result, the LIGO detectors' four kilometer tubes would only shift by about four billionths of a nanometer when one passed. The other problem is that these events are extremely rare, with estimates that only one occurs in the average galaxy over about 10,000 years.

Both of those issues are now being addressed. Using the lessons learned from the first detector, work is underway on an Advanced LIGO that may be operational later this year. But increased sensitivity will only get us so far if the events are rare. The solution there is to look deeper into the Universe, and to do that requires more detectors. Europe already has one in VIRGO, located outside of Pisa. These detectors will eventually be joined by LIGO-India and the KAGRA detector in Japan. With three detectors in operation, their combined sensitivity will let us sense events out to about a billion light years; with all five, our reach will extend out to nearly 2.5 billion. (For context, Andromeda, the nearest galaxy, is 2.5 million light years away.)

By 2017, we should be able to detect about 10 events a year. Perhaps just as importantly, we'll have a better sense of where the events we see are taking place. With only three detectors, the errors on locating events would be about 50 degrees; with all five, that figure drops to six degrees. This result might give us a chance to point a telescope at it to see what it looks like.

Of course, to some extent, we have to know what we're looking for, and in the case of a merger of black holes, that's not necessarily obvious. The author of the perspective, Mansi Kasliwal, indicates that theoretical physicists have been busy with trying to understand the sort of energy that might be released. Initial models put the optical output of neutron star mergers somewhere in between a nova and a supernova and suggest that they will emit reddish light for hours or days. That may be enough to allow an infrared survey telescope to spot one.

For now, gravity waves remain rooted firmly in theory, with only indirect evidence for their existence. But if all goes according to plan, that status will have changed by the end of the decade.

71 Reader Comments

It's absolutely amazing that after decades of work, we still seem to be stuck at "maybe LIGO will find something soon...". I understand the difficulty, but this has been an amazingly elusive thing to pin down. But very important; I mean, if we get to 2017 and STILL nothing, some serious crap will start hitting some theoretical fans.

It's absolutely amazing that after decades of work, we still seem to be stuck at "maybe LIGO will find something soon...". I understand the difficulty, but this has been an amazingly elusive thing to pin down. But very important; I mean, if we get to 2017 and STILL nothing, some serious crap will start hitting some theoretical fans.

I don't think past detectors were really expected to spot anything. They could have, if we got lucky and had a strong source pop up nearby or if there was an unexpectedly strong background, but we'd have been very lucky to have such a thing actually happen. It was still worth trying on its own merits, and a required step in order to develop the technology.

It's absolutely amazing that after decades of work, we still seem to be stuck at "maybe LIGO will find something soon...". I understand the difficulty, but this has been an amazingly elusive thing to pin down. But very important; I mean, if we get to 2017 and STILL nothing, some serious crap will start hitting some theoretical fans.

the problem is, as John mentioned, that signals strong enough for the current generation of sensors to detect are extremely rare. Transient merger signals at detection level are thousands of years apart. The situation isn't much better; there are no known binary neutron stars/stellar mass black holes close enough to us and each other to be generating an CW in-spiral signal strong enough to detect. While "mountains" (at most a few meters high) on a neutron star will generate a CW signal as they slow the stars spin only one known ns is close enough to potentially generate a signal. However without knowing the heights and orientations relative to the spin axis it's impossible to know how strong the signal actually is. In a set of posters the LIGO team did several years ago they put +-2 order of magnitude error bars on the potential spindown signals from the nearest known pulsars. Advanced LIGO should be able to easily detect several of them; but only the crab nebula pulsar is a potential target with the current hardware; the middle point of its error bars was roughly on the design sensitivity curve for LIGO.

LIGO and all the previous efforts have been learning how to build more sensitive hardware and, hoping we'd get lucky and either have an neutron star much closer by than any of the known ones or get lucky and have a merger event take place really close by. ("The most likely time for a one in a million event to take place is next Tuesday"; doesn't only apply to the impossible case you decided didn't need to be defended against.)

Advanced LIGO will be the big payoff; either GW signals are detected in the first few years or something is wrong with at least one of our understandings of post-stellar evolution or general relativity. In either case several people involved will almost certainly find themselves as Guests of Honor in Stockholm for the results.

It's absolutely amazing that after decades of work, we still seem to be stuck at "maybe LIGO will find something soon...". I understand the difficulty, but this has been an amazingly elusive thing to pin down. But very important; I mean, if we get to 2017 and STILL nothing, some serious crap will start hitting some theoretical fans.

the problem is, as John mentioned, that signals strong enough for the current generation of sensors to detect are extremely rare. Transient merger signals at detection level are thousands of years apart. The situation isn't much better; there are no known binary neutron stars/stellar mass black holes close enough to us and each other to be generating an CW in-spiral signal strong enough to detect. While "mountains" (at most a few meters high) on a neutron star will generate a CW signal as they slow the stars spin only one known ns is close enough to potentially generate a signal. However without knowing the heights and orientations relative to the spin axis it's impossible to know how strong the signal actually is. In a set of posters the LIGO team did several years ago they put +-2 order of magnitude error bars on the potential spindown signals from the nearest known pulsars. Advanced LIGO should be able to easily detect several of them; but only the crab nebula pulsar is a potential target with the current hardware; the middle point of its error bars was roughly on the design sensitivity curve for LIGO.

LIGO and all the previous efforts have been learning how to build more sensitive hardware and, hoping we'd get lucky and either have an neutron star much closer by than any of the known ones or get lucky and have a merger event take place really close by. ("The most likely time for a one in a million event to take place is next Tuesday"; doesn't only apply to the impossible case you decided didn't need to be defended against.)

Advanced LIGO will be the big payoff; either GW signals are detected in the first few years or something is wrong with at least one of our understandings of post-stellar evolution or general relativity. In either case several people involved will almost certainly find themselves as Guests of Honor in Stockholm for the results.

I think it also just underlines how incredibly weak gravity is, even though most people erroneously think of it as the strongest "force" that they experience daily.

Well, we don't want to get too lucky when it comes to nearby neutron stars..

"Advanced LIGO will be the big payoff; either GW signals are detected in the first few years or something is wrong with at least one of our understandings of post-stellar evolution or general relativity."

I think it also just underlines how incredibly weak gravity is, even though most people erroneously think of it as the strongest "force" that they experience daily.

Well, if you look at it on an atomic level sure, gravity is extremely weak, but it also it cumulitive. I've always been fascinated that an atom at the earths core, as weak as it is, in conjunction with all the other atoms of the earth, exert the gravitational force that keeps us on the ground. Of course my grasp of physics is rudimentary so I could be completely wrong how that works. Not my field... no pun intended, I'm a wetlands scientist, but I thought that while the force is weak, its effect extends quite far, which is how we can even get matter to coalesce.

I have a question here. Isn't there a serious assumption being made here, and that is that the tubes absolutely remain straight at all times? Distortions could come from things such as earthquakes, and maybe other things. Maybe they can account for that, but it did seem obvious that that was an underlying assumption.

I have a question here. Isn't there a serious assumption being made here, and that is that the tubes absolutely remain straight at all times? Distortions could come from things such as earthquakes, and maybe other things. Maybe they can account for that, but it did seem obvious that that was an underlying assumption.

Earthquakes can be measured in ways that wouldn't be sensitive to gravity waves then subtracted from the signal.

@urkle: your question is totally legitimate, and in fact the problem is far worse than that! Earthquakes are definitely a problem, so they need some sort of active stabilization for that... but it's not just earthquakes, it's all vibrations caused in the vicinity of the instruments as well. Even trucks rolling down a nearby highway will cause a measurable signal if they don't have their stabilization correct. The engineering involved is seriously impressive!

However, the problem is so bad when we are getting to these scales (attometer interference over 4 kilometer distance??) it is hard for me to imagine how they could convince themselves they have beat the problem in a single detector. When they have a number of detectors though, I can see how a signal might be teased out.

Beyond these considerations though, my question would be: how do they know their clever stabilization strategies might not cancel out the signal itself?

Thinking about it more, at attometer scales, quantum effects would start to be a big problem in addition to stray particles hitting the emitters or detector, thermal electrons pushing the thing around, etc...

I don't even know enough to imagine what the problems would be on that scale, let alone how you engineer around them!

Well, we don't want to get too lucky when it comes to nearby neutron stars..

"Advanced LIGO will be the big payoff; either GW signals are detected in the first few years or something is wrong with at least one of our understandings of post-stellar evolution or general relativity."

I don't see that. How so?

Mistakes help to eliminate erroneous logic and help to refine solutions. It's basic troubleshooting and problem solving.

Well, we don't want to get too lucky when it comes to nearby neutron stars..

"Advanced LIGO will be the big payoff; either GW signals are detected in the first few years or something is wrong with at least one of our understandings of post-stellar evolution or general relativity."

I don't see that. How so?

Mistakes help to eliminate erroneous logic and help to refine solutions. It's basic troubleshooting and problem solving.

That is deep man. I don't get it.

The advanced one failing to detect a wave (or detecting one) won't change anything about general relativity, at least that I'm aware of.

I have a few questions. What particle mediates or transports the wave? Or how does it travel? We can see the effects, but how does that energy reach us?

In general relativity, it's a ripple in space. Space is tugged back and forth by some astrophysical object, like a binary black hole system, and ripples propagate away just as when you shake a cork floating in water. The quantum mechanical interpretation is that the system is emitting polarized gravitons.

I have a few questions. What particle mediates or transports the wave? Or how does it travel? We can see the effects, but how does that energy reach us?

That's an open question.

Cosmologists tend to lean towards the idea that gravity is quantized (referred to as a graviton), mainly because all the other forces are. There are formulations of general relativity with quantized gravity, but they all have flaws.

Gravity seems to work so much differently than the other forces, though, that it may be possible that it is not quantized. In that case, gravity waves would propagate through space as pure classical-like waves in the "material" of space-time.

Unlike with electromagnetic waves, I do not believe we have found any phenomenon in nature that would be inconsistent with a classical formulation of gravity waves. For example, in EM, even before a quantum theory was developed, we knew there was something wrong with a classical formulation of EM waves for various reasons, including the ultraviolet catastrophe.

I have a few questions. What particle mediates or transports the wave? Or how does it travel? We can see the effects, but how does that energy reach us?

That's an open question.

Cosmologists tend to lean towards the idea that gravity is quantized (referred to as a graviton), mainly because all the other forces are. There are formulations of general relativity with quantized gravity, but they all have flaws.

Gravity seems to work so much differently than the other forces, though, that it may be possible that it is not quantized. In that case, gravity waves would propagate through space as pure classical-like waves in the "material" of space-time.

Unlike with electromagnetic waves, I do not believe we have found any phenomenon in nature that would be inconsistent with a classical formulation of gravity waves. For example, in EM, even before a quantum theory was developed, we knew there was something wrong with a classical formulation of EM waves for various reasons, including the ultraviolet catastrophe.

Maybe I'm wrong, but I'd tend to think that it's easier to discover inconsistent phenomenon in EM than in gravitation given that we still haven't detected a single gravitational wave. For example, in the case of newtonian gravity, it is my understanding that even before anything remotely related to general gravity existed, it was pretty obvious that many observations even right in our galactic backyard (solar system) didn't add up.

I'm an undergraduate trying to get a degree in engineering, and my physics courses barely touched these subjects, but I've always thought that general relativity was a step in between actual understanding of gravity. We can describe gravity's effect, but how gravity comes to be, is still a mistery.

I understand that my original question was a bit open-ended, because those questions most likely don't have an answer yet, but if we are investing heavily into trying to detect these waves, we have to have at least a supposition, of how they would be carried away. I just can't grasp the space-time just rippling through. But maybe it's just me.

Well, we don't want to get too lucky when it comes to nearby neutron stars..

"Advanced LIGO will be the big payoff; either GW signals are detected in the first few years or something is wrong with at least one of our understandings of post-stellar evolution or general relativity."

I don't see that. How so?

Mistakes help to eliminate erroneous logic and help to refine solutions. It's basic troubleshooting and problem solving.

That is deep man. I don't get it.

The advanced one failing to detect a wave (or detecting one) won't change anything about general relativity, at least that I'm aware of.

Advanced LIGO's design sensitivity is high enough that the only reasons for not it detecting a signal are: general relativity being wrong and the signal we're expecting to detect not existing; or the evolution of stellar remnants/galaxies being wrong and there not being anything to generate the signals we're listening for in existence.

I have a few questions. What particle mediates or transports the wave? Or how does it travel? We can see the effects, but how does that energy reach us?

That's an open question.

Cosmologists tend to lean towards the idea that gravity is quantized (referred to as a graviton), mainly because all the other forces are. There are formulations of general relativity with quantized gravity, but they all have flaws.

Gravity seems to work so much differently than the other forces, though, that it may be possible that it is not quantized. In that case, gravity waves would propagate through space as pure classical-like waves in the "material" of space-time.

Unlike with electromagnetic waves, I do not believe we have found any phenomenon in nature that would be inconsistent with a classical formulation of gravity waves. For example, in EM, even before a quantum theory was developed, we knew there was something wrong with a classical formulation of EM waves for various reasons, including the ultraviolet catastrophe.

Maybe I'm wrong, but I'd tend to think that it's easier to discover inconsistent phenomenon in EM than in gravitation given that we still haven't detected a single gravitational wave. For example, in the case of newtonian gravity, it is my understanding that even before anything remotely related to general gravity existed, it was pretty obvious that many observations even right in our galactic backyard (solar system) didn't add up.

I'm an undergraduate trying to get a degree in engineering, and my physics courses barely touched these subjects, but I've always thought that general relativity was a step in between actual understanding of gravity. We can describe gravity's effect, but how gravity comes to be, is still a mistery.

I understand that my original question was a bit open-ended, because those questions most likely don't have an answer yet, but if we are investing heavily into trying to detect these waves, we have to have at least a supposition, of how they would be carried away. I just can't grasp the space-time just rippling through. But maybe it's just me.

That is harder to grasp than electromagnetic waves rippling by while quantized into packets?

I have a question here. Isn't there a serious assumption being made here, and that is that the tubes absolutely remain straight at all times? Distortions could come from things such as earthquakes, and maybe other things. Maybe they can account for that, but it did seem obvious that that was an underlying assumption.

Well, for the VIRGO detector, each optical component is isolated from seismic noise with 10 metre tall system of compound pendulums. The beam also travels in a high vacuum, so that atmosphere inside wouldn't influence the measurements.

Well, we don't want to get too lucky when it comes to nearby neutron stars..

"Advanced LIGO will be the big payoff; either GW signals are detected in the first few years or something is wrong with at least one of our understandings of post-stellar evolution or general relativity."

I don't see that. How so?

Mistakes help to eliminate erroneous logic and help to refine solutions. It's basic troubleshooting and problem solving.

That is deep man. I don't get it.

The advanced one failing to detect a wave (or detecting one) won't change anything about general relativity, at least that I'm aware of.

If there really aren't any gravity waves, it will mean that GR predicted something that turned out not to be there, even though the effect (loss of energy in binary star systems) is. That, in turn, means that either we are confused about how stars work, or there is a hole in GR.

I experience the other forces every time my fingers don't slip right through the handle on my coffee cup. I'm guessing everyone else does, too. :-)

That's exclusively electromagnetism. Our experience of the weak force is limited to radioactive decay of unstable particles in our bodies and the environment. I can't think of a way we experience the strong force, other than being pelted by the occasional cosmic ray hadron, if you count that. The other forces are active on the femtometer scale and below, so they just don't have much opportunity to show up.

I have a question here. Isn't there a serious assumption being made here, and that is that the tubes absolutely remain straight at all times? Distortions could come from things such as earthquakes, and maybe other things. Maybe they can account for that, but it did seem obvious that that was an underlying assumption.

Well, as Antrozous said above, there's an awful lot of isolation techniques being employed.

Plus with multiple detectors widely geographically distributed, the chances of multiple earthquakes hitting each of them with vibrations of the same magnitude in the same x,y,z direction at the same time (with appropriate speed of light delays) is as near to zero as makes no odds, then on top of that the expected spectra and durations are different.

Thinking about it more, at attometer scales, quantum effects would start to be a big problem in addition to stray particles hitting the emitters or detector, thermal electrons pushing the thing around, etc...

I don't even know enough to imagine what the problems would be on that scale, let alone how you engineer around them!

Part of it is that any gravity wave will be at a single, well-defined frequency, so you can use techniques similar to a lock-in amplifier to see stuff that's truly periodic. Thermal noise, of course, is not periodic. Some frequency ranges are in fact completely impossible to see a wave in, because you're swamped by backgrounds (e.g., the normal seismic groaning of the Earth) but these have well known frequencies, and you look outside those regions.

It's absolutely amazing that after decades of work, we still seem to be stuck at "maybe LIGO will find something soon...". I understand the difficulty, but this has been an amazingly elusive thing to pin down. But very important; I mean, if we get to 2017 and STILL nothing, some serious crap will start hitting some theoretical fans.

I don't think that observing gravity waves will ever be possible on the ground, there's just too much noise (or not enough signal). But a space-based instrument will likely never be green-lit unless we have some proof, keeping everything in a sort of vicious cycle of incrementally improving the ground based instruments. Such a waste of resources that multiple countries get their own version of the just-about-but-not-quite-sensitive-enough detector, we probably could've made a beautiful array of satellites by now.

"... the LIGO detectors' four kilometer tubes would only shift by about four billionths of a nanometer when one passed" - actually we [I and my colleagues in LSC] don't care about the tubes, we're looking for a displacement of the suspended, seismically isolated mirrors.

"To increase sensitivity, the LIGO project also operated two detectors," .. no, the two detectors are expected to see approximately the same signal (they 'listen' to the same part of the sky), so operating two of them doesn't affect the basic limit of sensitivity. There are two in order to avoid mistaking a weird noise event at a single detector for a GW signal, and so to increase confidence in a possible detection.

As for the merger of black holes being "in between a nova and a supernova" or "emitting reddish light for hours or days" ... well, someone ought to learn the difference between black holes and neutron stars. The article by Kasliwal is, unfortunately, also unclear on this, since it doesn't bother to distinguish between mergers of neutron stars and black holes. Only those involving at least one neutron star are expected to emit electromagnetic energy (i.e. radio/light/X-rays/gamma-rays). Black hole mergers probably don't emit any light at all, so they are only visible via gravitational waves.

The author also seems to be confused about the energy "release" in mergers: I guess what he means is electromagnetic energy, but in fact the energy released in the form of gravitational waves is expected to be hugely greater than electromagnetic energy. One binary black hole merger could, for a fraction of a second, emit more power in gravitational waves than the light from an entire galaxy...

The advanced one failing to detect a wave (or detecting one) won't change anything about general relativity, at least that I'm aware of.

Advanced LIGO's design sensitivity is high enough that the only reasons for not it detecting a signal are: general relativity being wrong and the signal we're expecting to detect not existing; or the evolution of stellar remnants/galaxies being wrong and there not being anything to generate the signals we're listening for in existence.

You left out number 3. And 4 even.

Number 1, GR being "wrong", that's doubtful. Gravity certainly does take energy away from mass.

And number 2, stellar evolution, we're not "right" about it now. I mean, those models are going to get continually refined; Mr. Spock takes readings on interesting blue giants, so there will always be something to learn. Actually, it could turn out that this experiment contributes something to this field.

Number 3, gravity takes energy away from a system just like they think it does, but not in a detectable wave. That last part is the tip end of the theory anyway. Ah, if that's what you meant by number 2, well ok. But it would hardly change everything.

Number 4, machine is broken.

I don't get how the hell it's supposed to work anyway. I can read, but dang. You're talking about extra gravity, with no mass or energy to account for it; it's like a dimensional wave, passing through the universe.. at what speed? From your perspective, it's going to affect the entire universe simultaneously, and then move on; so I guess you've got the conflicting histories of the two detectors...

And these things are right next to each other. I'd think you'd want them as far apart as possible. These are (theorized to be) waves, so they would travel; increasing the distance between probes increases the resolution. Trouble is, you have to shine them together at some point, due to the design.

It's a hell of an attempt at a machine, I'll give them that, but it not working, could be from a number of reasons, and won't change anything, in my opinion. They would just build yet another. Maybe that one uses neutrinos, and the companion neutrino laser thing is on Mars. And kids have models of this machine in their Lincoln Log sets.

If the gravity wave is a ripple in space-time, are we sure that it would not affect the speed of light, thereby making the movement in the detectors undetectable by interference?

I must admit this article left a nagging hole that I think disturbs some other posters as well. I really don't get what a gravity wave is, and how it relates to the force that we all experience, that holds clumps of matter together, and that keeps us from flying off on a rogue planet. I don't get how such a significant cosmic force can be carried by virtually undetectable waves.

I think it also just underlines how incredibly weak gravity is, even though most people erroneously think of it as the strongest "force" that they experience daily.

Well, if you look at it on an atomic level sure, gravity is extremely weak, but it also it cumulitive. I've always been fascinated that an atom at the earths core, as weak as it is, in conjunction with all the other atoms of the earth, exert the gravitational force that keeps us on the ground. Of course my grasp of physics is rudimentary so I could be completely wrong how that works. Not my field... no pun intended, I'm a wetlands scientist, but I thought that while the force is weak, its effect extends quite far, which is how we can even get matter to coalesce.

Indeed. We have 4 forces, the Strong force operates only over VERY small distances due to its nature, the weak force is also short range and invisible to most matter. The electromagnetic force is long-range, but due to being both repulsive and attractive on net always balances itself out (though it definitely can have large scale effects, solar flares are a good example). Only gravity is fully attractive, felt by everything in the universe, and thus just builds up over large scales to dominate the overall structure of the universe.

If the gravity wave is a ripple in space-time, are we sure that it would not affect the speed of light, thereby making the movement in the detectors undetectable by interference?

I must admit this article left a nagging hole that I think disturbs some other posters as well. I really don't get what a gravity wave is, and how it relates to the force that we all experience, that holds clumps of matter together, and that keeps us from flying off on a rogue planet. I don't get how such a significant cosmic force can be carried by virtually undetectable waves.

Gravity is a mathematically ugly thing (at least GR is, any replacement would have to be even MORE ugly). GR doesn't talk about 'ripples in spacetime', that would imply that there was some sort of absolute metric OUTSIDE of spacetime, but in fact there is no such thing, that's the entire fundamental point of GR! We could talk about stress-energy tensors and relativistic action, but I can promise you 99.999% of the people on this -or any- forum are not going to get anything very useful out of that.

Gravity isn't 'carried' by gravity waves, they are merely a form of energy which has a gravitational effect. The gravity between the Earth and the Sun for instance is not 'carried by gravity waves', it simply exists by virtue of the way the mass of the Sun and the Earth affect the stress-energy tensor. In terms of GR gravity isn't a 'force', it is simply the consequence of the way spacetime works. The difference is relatively hair-splitting, but in a classical sense there's a large difference between the way EM works (all energy is bound into EM fields which propagate through spacetime) and gravity which simply IS the shape of spacetime.

If the gravity wave is a ripple in space-time, are we sure that it would not affect the speed of light, thereby making the movement in the detectors undetectable by interference?

I must admit this article left a nagging hole that I think disturbs some other posters as well. I really don't get what a gravity wave is, and how it relates to the force that we all experience, that holds clumps of matter together, and that keeps us from flying off on a rogue planet. I don't get how such a significant cosmic force can be carried by virtually undetectable waves.

Gravity is a mathematically ugly thing (at least GR is, any replacement would have to be even MORE ugly). GR doesn't talk about 'ripples in spacetime', that would imply that there was some sort of absolute metric OUTSIDE of spacetime, but in fact there is no such thing, that's the entire fundamental point of GR! We could talk about stress-energy tensors and relativistic action, but I can promise you 99.999% of the people on this -or any- forum are not going to get anything very useful out of that.

Gravity isn't 'carried' by gravity waves, they are merely a form of energy which has a gravitational effect. The gravity between the Earth and the Sun for instance is not 'carried by gravity waves', it simply exists by virtue of the way the mass of the Sun and the Earth affect the stress-energy tensor. In terms of GR gravity isn't a 'force', it is simply the consequence of the way spacetime works. The difference is relatively hair-splitting, but in a classical sense there's a large difference between the way EM works (all energy is bound into EM fields which propagate through spacetime) and gravity which simply IS the shape of spacetime.

But if it is the shape, and that shape changes, then wouldn't that shape changing make waves?

If the gravity wave is a ripple in space-time, are we sure that it would not affect the speed of light, thereby making the movement in the detectors undetectable by interference?

I must admit this article left a nagging hole that I think disturbs some other posters as well. I really don't get what a gravity wave is, and how it relates to the force that we all experience, that holds clumps of matter together, and that keeps us from flying off on a rogue planet. I don't get how such a significant cosmic force can be carried by virtually undetectable waves.

Gravity is a mathematically ugly thing (at least GR is, any replacement would have to be even MORE ugly). GR doesn't talk about 'ripples in spacetime', that would imply that there was some sort of absolute metric OUTSIDE of spacetime, but in fact there is no such thing, that's the entire fundamental point of GR! We could talk about stress-energy tensors and relativistic action, but I can promise you 99.999% of the people on this -or any- forum are not going to get anything very useful out of that.

Gravity isn't 'carried' by gravity waves, they are merely a form of energy which has a gravitational effect. The gravity between the Earth and the Sun for instance is not 'carried by gravity waves', it simply exists by virtue of the way the mass of the Sun and the Earth affect the stress-energy tensor. In terms of GR gravity isn't a 'force', it is simply the consequence of the way spacetime works. The difference is relatively hair-splitting, but in a classical sense there's a large difference between the way EM works (all energy is bound into EM fields which propagate through spacetime) and gravity which simply IS the shape of spacetime.

I understood that you had to add string theory to the mix for gravity to be a natural consequence, within the framework of GR and the standard model I thought gravity was still unexplained.

I have a few questions. What particle mediates or transports the wave? Or how does it travel? We can see the effects, but how does that energy reach us?

That's an open question.

Cosmologists tend to lean towards the idea that gravity is quantized (referred to as a graviton), mainly because all the other forces are. There are formulations of general relativity with quantized gravity, but they all have flaws.

Gravity seems to work so much differently than the other forces, though, that it may be possible that it is not quantized. In that case, gravity waves would propagate through space as pure classical-like waves in the "material" of space-time.

Unlike with electromagnetic waves, I do not believe we have found any phenomenon in nature that would be inconsistent with a classical formulation of gravity waves. For example, in EM, even before a quantum theory was developed, we knew there was something wrong with a classical formulation of EM waves for various reasons, including the ultraviolet catastrophe.

I understood that you had to add string theory to the mix for gravity to be a natural consequence, within the framework of GR and the standard model I thought gravity was still unexplained.

You are right though it's tensors all the way down.

In GR 'gravitational attraction' is simply systems following the natural path of stationary action through their state spaces. This is usually explained in terms of 'curved spacetime' and world lines within that manifold, so for example the Earth isn't 'attracted to the Sun by gravity', it simply follows a path of stationary action, which in space is a elipse. Its not 'incorrect' to think about curves in spacetime and waves of gravity, but we have to be careful not to let it confuse our thinking with ideas of some sort of external absolute metric, which this kind of statement tends to evoke. It is better to think of the stress-energy tensor as a field with a value at all points in space-time.

Anyway, the point is gravity, as a sensed force of attraction between masses, DOES fall out of GR naturally. At some level NOTHING is ever entirely explained. String Theory may provide some more complete explanation, but without making unique predictions it actually has so far no explanatory power. Currently string theories are just wool gathering as they don't predict anything.

I have a few questions. What particle mediates or transports the wave? Or how does it travel? We can see the effects, but how does that energy reach us?

That's an open question.

Cosmologists tend to lean towards the idea that gravity is quantized (referred to as a graviton), mainly because all the other forces are. There are formulations of general relativity with quantized gravity, but they all have flaws.

Gravity seems to work so much differently than the other forces, though, that it may be possible that it is not quantized. In that case, gravity waves would propagate through space as pure classical-like waves in the "material" of space-time.

Unlike with electromagnetic waves, I do not believe we have found any phenomenon in nature that would be inconsistent with a classical formulation of gravity waves. For example, in EM, even before a quantum theory was developed, we knew there was something wrong with a classical formulation of EM waves for various reasons, including the ultraviolet catastrophe.

I'm curious, what ultraviolet catastrophe ?

Quantization explains why black bodies don't simply emit all their thermal energy at the highest possible frequency instantly. This was the ultimate nut that classical physics couldn't crack. If EM radiation can exist at any arbitrary energy then this 'ultraviolet catastrophe' would be the result. Instead Planke -amongst others including Einstein- invented a new law which explained observation in terms of quantized energy levels, preventing higher modes that would have infinite power from existing at all.